There were sport model 2 and 4 doors, but in the USA e36 range 'is' denotes 2door. In all other model ranges (e30, etc) the 'is' means sport.

In europe is means sport, as there were both 318/325/328i coupes and sedans with sports models being denoted with 's'.

Obviously bmw further mixed things up with using different engines under the same name, and using faux naming conventions (like 323i which actually uses a 2.5L engine but was named differently for political reasons to further the 2.5l single vanos engine from the 2.8l dual vanos 196hp 328i vehicle..also bmw historically always makes models with at least a '5' between each model... 318/325 were produced concurrently.. 323/328 were produced concurrently... 330/325 were produced concurrently.. 325i/330i / 328ci/335ci (more then 5)... etc etc. i guess 5 is a satisfying number)

this was all off the top of my head at 2:30 in the morning, but im pretty sure its mostly accurate

This explains the differences between my car and my brother's. I've heared that 318is is 16 valves with double camshafts. But in all the forums, the guys just say that 318i means 4 doors and 318is means 2 doors which didn't really convince me.

When I used to have E30 before, I saw some cars with "is" exetension and I was told they're with "sports" package. This means that BMW should follow the same convention for E36.